The present invention relates generally to an accessory mounting assembly for electrical distribution equipment. More specifically, the present invention relates to an accessory mounting assembly for supporting internal lightning arrestors and/or additional fuses such as expulsion or weak-link fuses.
Distribution transformers are utilized to step voltage levels up or down in electrical power distribution systems. Stepping up the voltage levels of the electrical power reduces the current required to carry that power and minimizes line losses. However, a great deal of heat is emitted when the power passes through the transformers, therefore the transformers are often cooled by being immersed in containers filled with a cooling liquid such as oil. This is especially true when high power is involved.
Electrically connected to the transformers are fuses which provide various forms of current protection, e.g., over current protection, current limiting protection, and energy limiting protection, for the distribution transformers and the distribution system as a whole, particularly against lightning strikes. Fuse holders are utilized for supporting these fuses. The fuses and fuse holders are often mounted to the distribution transformers themselves and immersed in the same cooling liquid.
One type of device that is held by a fuse holder is a lightning arrester. When lightning strikes a transformer or near one, a lightning arrester can provide a discharge path to a ground. Typically, each lightning arrester is retained by a holder that consumes a large volume within a transformer.
Generally, fuse holders for distribution transformers generally contain a large number of bulky parts, and are therefore costly to assemble and difficult to manufacture to desired quality standards. Moreover, these fuse holders do not have a standard design. Many different types of fuse holders are utilized to accommodate the various fuses connected to the distribution transformers.
Additionally, the cooling oil presents a harsh environment to the fuses and fuse holders. This oil is subject to extreme temperature ranges, e.g., from -30 degrees centigrade to 140 degrees centigrade. Therefore expansion and contraction of the fuses and fuse holders must be taken into account in order to maintain a safe distance between fuses. However, this adds to the undesirable size, complexity, and variety of the fuse holders.
Additionally adding to the size of the fuse holders is the requirement to maintain a proper creep distance between the fuses. In high voltage multi-phase situations, e.g., fuses electrically connected to the three phases of a distribution transformer, a significant voltage differential will develop between the ends of the fuses protecting each phase. This voltage differential will cause a current (creep current) to conduct on the surface of the fuse support between the ends of the fuses mounted on a fuse holder if the fuses are not positioned a safe distance (creep distance) apart. The requirement to maintain a safe creep distance between each fuse mounted on a fuse holder significantly increases the overall size of the fuse holder even if the fuse holder is comprised of non-conductive material.
Furthermore, the containers into which the fuses, fuse holders and transformers are immersed have a limited capacity, and the wire cables used in the installation of this type of equipment are bulky. Therefore, wire routing around the fuse holders is difficult in the limited space available.